Field of Invention
The present invention relates to a passivation layer on a semiconductor light emitting device.
Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
U.S. 2006/0281203 describes “techniques for mounting LEDs for packaging and for removing the growth substrate of the LEDs.” An underfill material is injected between the carrier and die to provide support to the epitaxial structure during substrate removal. A portion of the semiconductor structure is supported by the interconnects between the die and the carrier, and a portion is supported by the underfill. To prevent cracking when the semiconductor structure is exposed to stress, for example during substrate removal, the mechanical compliance and coefficient of thermal expansion of the interconnects and the underfill are preferably matched. Examples of suitable underfill materials include FB4511 epoxy available from Henkel Corporation, and silicones and other epoxies loaded with inorganic materials such as silica or alumina to reach the desired coefficient of thermal expansion and mechanical compliance. Since the underfill provides support for the epitaxial layers, it is desirable for the underfill to fill all gaps between the interconnects and to avoid the formation of air bubbles which may encourage cracking of the epitaxial structure during substrate removal. Accordingly, the surface tension of the underfill material may be selected such that the underfill fills all gaps between the interconnects. Alternatively, a partial vacuum may be created on a side of the gap between the carrier and the die opposite the side where the underfill is injected, to draw the underfill into all gaps between the interconnects.